Thursday, May 29, 2008 - 9:45 AM
509

The fate of assimilated 15N-nitrate in 71 headwater streams

Laura T. Johnson1, Jennifer L. Tank1, Patrick J. Mulholland2, Robert O. Hall3, Stuart Findlay4, William H. McDowell5, H. Maurice Valett6, Stephen K. Hamilton7, Bruce J. Peterson8, Linda R. Ashkenas9, Lee W. Cooper10, Clifford N. Dahm11, Walter K. Dodds12, Stanley V. Gregory9, Nancy B. Grimm13, Sherri L. Johnson14, Judith L. Meyer15, Geoffrey C. Poole16, Jackson R. Webster17, Clay Arango18, Jake J. Beaulieu19, Melody J. Bernot20, Amy J. Burgin21, Chelsea L. Crenshaw22, Ashley M. Helton15, Bobbie R. Niederlehner23, Jonathan M. O'Brien7, Jody D. Potter5, Richard W. Sheibley24, Daniel J. Sobota25, and Suzanne M. Thomas8. (1) Department of Biological Sciences, University of Notre Dame, Galvin Life Sciences, Notre Dame, IN 46556, (2) Environmental Sciences Division, Oak Ridge National Laboratory, Bethel Valley Road, PO Box 2008, Oak Ridge, TN 37831-6036, (3) Department of Zoology and Physiology, University of Wyoming, Laramie, WY 82071, (4) Inst. of Ecosystem Studies, Millbrook, NY 12545, (5) Department of Natural Resources, University of New Hampshire, Durham, NH 03824, (6) Biological Sciences, Virginia Polytechnic Institute and State University, 2119 Derring Hall, Blacksburg, VA 24061, (7) Kellogg Biological Station, 3700 East Gull Lake Drive, Hickory Corners, MI 49060, (8) Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA 02543, (9) Department of Fisheries and Wildlife, Oregon State University, Corvallis, OR 97331, (10) Department of Ecology and Evolutionary Biology, The University of Tennessee, Knoxville, TN 37932, (11) Department of Biology, University of New Mexico, MSC03 2020, Albuquerque, NM 87131, (12) Division of Biology, Kansas State University, 104 Ackert Hall, Manhattan, KS 66506, (13) School of Life Sciences, Arizona State University, Tempe, AZ 85287, (14) Pacific NW Research Station, US Forest Service, 3200 SW Jefferson Way, Corvallis, OR 97331, (15) Odum School of Ecology, University of Georgia, Athens, GA 30602, (16) Dept. of Land Resources and Environmental Sciences, Montana State University, 819 Leon Johnson Hall, P.O. Box 173120, Bozeman, MT 59717, (17) Department of Biological Sciences, Virginia Tech, Blacksburg, VA 24061, (18) Depts of Biological Sciences/Geography, Central Washington University, Science Building, Room 236A, 400 E. University Way, Ellensburg, WA 98926-7537, (19) US EPA, 26 W. Martin Luther King Drive, Cincinnati, OH 45268, (20) Department of Biology, Ball State University, Muncie, IN 42306, (21) Cary Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545, (22) Department of Biology, Utah State University, Logan, UT 84322, (23) Department of Biological Sciences, Virginia Polytechnic Institute and State University, 2119 Derring Hall, Blacksburg, VA 24061, (24) Washington Water Science Center, U. S. Geological Survey, Tacoma, WA 98402, (25) School of Earth and Environmental Sciences, Washington State University, Vancouver Campus, Vancouver, WA 98686

Streams retain nitrogen (N) via denitrification and assimilatory uptake, but assimilation may retain N only temporarily. We examined the short-term fate of assimilated nitrate in 71 streams with differing land use in eight US biomes.  Assimilated N can be retained in biomass, released as dissolved organic nitrogen (DON) or ammonium, or transported downstream as suspended particulate N (SPON).  Using a mass balance approach with data from the 24h LINXII 15N-nitrate tracer additions, we quantified downstream export of DO15N, 15N-ammonium, and SPO15N compared to assimilated 15N in biomass compartments. Relative to nitrate removal via denitrification, 19% of streams had higher DON export, 8% had higher ammonium export, but none had higher SPON export.  The fraction of assimilated nitrate lost as DON and ammonium was highly variable (DON=0-55%, ammonium=0-46%), but N lost as SPON was always low (<<1%). In >60% of 71 streams, DON release and ammonification represented <1% of assimilated nitrate.  However, in 5 streams DON release was >30% and ammonification was >14% of assimilated N. Although DON and ammonium export were frequently a small fraction of assimilated nitrate, some headwater streams rapidly cycled nitrate through assimilatory pathways to yield relatively high export of DON and ammonium compared to denitrification.


Web Page: land use, dissolved organic nitrogen, ammonium

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